1. Field of the Invention
The invention relates to an ohmic electrode for a p-type SiC semiconductor and a method of forming the same. More specifically, the invention relates to an ohmic electrode for a p-type SiC semiconductor, which has an ohmic electrode layer which is improved in surface smoothness and in which x, y, and z (z=1−x−y) in Ti(z)Si(x)C(y) are within a specific range, the ohmic electrode layer being directly laminated on the p-type SiC semiconductor, and a method of forming the same.
2. Description of the Related Art
A SiC single crystal has features of being thermally and chemically very stable, being excellent in mechanical strength, and being resistant to radiation. In addition, the SiC single crystal has excellent properties of being higher in, for example, dielectric breakdown voltage and thermal conductivity than a silicon (Si) single crystal. Doping the SiC single crystal with dopants forms a p-type or n-type conduction carrier, and the electronic characteristics of the carrier are easily adjusted by adjusting the manner of doping the SiC single crystal with the dopants. The SiC single crystal has a wide band gap (a 4H-type single crystal SiC has a band gap of approximately 3.3 eV, and a 6H-type single crystal SiC has a band gap of approximately 3.0 eV). Thus, a high temperature, a high frequency, a breakdown voltage, and an environment resistance, which cannot be achieved by an existing semiconductor material, for example, a Si single crystal or a gallium arsenide (GaAs) single crystal, can be achieved by the SiC single crystal. Therefore, expectations for the SiC single crystal as a next-generation semiconductor material have been raised.
On the other hand, it is known that practical application of a semiconductor device requires an electrode demonstrating good ohmic properties, namely, an ohmic electrode. The electrode demonstrating good ohmic properties means a low-resistance electrode that demonstrates current-voltage characteristics with a linear relationship (i.e., lacks nonlinearity) between a current and a voltage regardless of the direction of the current and the magnitude of the voltage, and that allows the current to flow smoothly in both directions. In the case of a p-type SiC semiconductor, however, there has not been so far established a technology for stably forming an ohmic electrode. Thus, various proposals have been made regarding the development of the ohmic electrode for the p-type SiC semiconductor.
For example, Japanese Patent Application Publication No. 1-20616 (JP-A-1-20616) describes a method in which Al and Si are sequentially laminated on a p-type SiC single crystal and a thermal treatment is thereafter carried out to form an ohmic electrode. More specifically, JP-A-1-20616 describes a method of forming an electrode for a p-type SiC semiconductor, in which a carrier concentration of the p-type SiC single crystal is set to a value equal to or higher than 1×1017/cm3 and a thermal treatment temperature is set to 400° C. to 500° C. Japanese Patent Application Publication No. 2003-86534 (JP-A-2003-86534) describes an ohmic electrode for a SiC semiconductor, which is formed by connecting an electrode to a first reaction layer that is formed on a p-type SiC semiconductor substrate in the form of a film through a thermal treatment and that contains C, Si, Al and a magnetic material forming an intermetallic compound with Si. JP-A-2003-86534 also describes a method of forming an ohmic electrode for a SiC semiconductor, which includes a first step of laminating an Al film and a Ni film on a surface of a p-type SiC semiconductor substrate, a second step of carrying out a thermal treatment in vacuum to form a first reaction layer, and a third step of connecting an electrode to the first reaction layer.
Japanese Patent Application Publication No. 2008-78434 (JP-A-2008-78434) describes a method of forming a semiconductor device that does not contain by-products such as Al4C3, Ti5Si3Cx and TiC. The method includes a first step of forming a Ti layer in contact with a SiC semiconductor layer and a second step of raising a temperature of the SiC semiconductor layer and the Ti layer to a temperature higher than a first reference temperature at which Ti and Al react with each other to produce Al3Ti and lower than a second reference temperature at which Al3Ti and SiC react with each other to produce Ti3SiC2 and forming an Al layer on the Ti layer. In the second step, SiC of the SiC semiconductor layer and Al3Ti react with each other to produce Ti3SiC2 and thus form a Ti3SiC2 layer in ohmic contact with the SiC semiconductor. However, the aforementioned publication does not describe the smoothness of a surface of the electrode after the thermal treatment.
Japanese Patent Application Publication. No. 2008-78435 (JP-A-2008-78435) describes a method of forming a semiconductor device that has a low contact resistance and does not contain by-products such as Al4C3, Ti5Si3Cx and TiC. The method includes a first step of forming a Ti layer in contact with a SiC semiconductor layer, a second step of forming an Al layer on the Ti layer, a third step of subjecting the SiC semiconductor layer, the Ti layer, and the Al layer to a thermal treatment at a temperature higher than a first reference temperature at which Ti and Al react with each other to produce Al3Ti and lower than a second reference temperature at which Al3Ti and SiC react with each other to produce Ti3SiC2 and thus forming an Al3Ti layer, and a fourth step of subjecting the SiC semiconductor layer and the Al3Ti layer to a thermal treatment at a temperature higher than the second reference temperature after completion of reaction forming Al3Ti from Ti and Al, to form a Ti3SiC2 layer in ohmic contact with the SiC semiconductor layer. However, the aforementioned publication does not describe the smoothness of a surface of the electrode after the thermal treatment.
Japanese Patent Application Publication No. 2008-227174 (JP-A-2008-227174) describes a method of forming an ohmic electrode on a P-type 4H—SiC substrate. This method includes a lamination step of sequentially laminating a first Al layer, a Ti layer, and a second Al layer, each of which has a thickness of 1 nm to 60 nm, on the P-type 4H—SiC substrate, and an alloying step of forming an alloy layer of the SiC substrate and the Ti layer using the first Al layer as a medium for reaction through a thermal treatment in a non-oxidizing atmosphere. However, the aforementioned publication does not describe the smoothness of a surface of the electrode after the thermal treatment. In addition, some of the aforementioned related art adopt a deposition-and-annealing (DA) method. According to the DA method, when an ohmic electrode for a p-type SiC semiconductor is formed, an Al deposited film and a Ti deposited film, which are actually unnecessary for the reaction, are formed in a laminated manner to form a Ti3SiC2 layer, and a thermal treatment is carried out at a temperature of approximately 1000° C. This thermal treatment leads to an interface reaction of the SiC used as a semiconductor material and Ti and Al deposited thereon and formation of a thin intermediate semiconductor layer that is made of Ti3SiC2 and that is in contact with the SiC semiconductor.
In the method of forming the electrode according to the related art, it is difficult to form an intermediate semiconductor layer made of Ti3SiC2 with a uniform thickness on an entire electrode portion on a SiC semiconductor, and compounds such as Al4C3, Ti5Si3Cx and TiC, Al3Ti are produced as by-products on an interface between the electrode portion and the intermediate semiconductor layer. Since an interface region where these by-products exist is high in contact resistance, it is difficult to reduce the contact resistance of the electrode on the SiC semiconductor to an appropriate ohmic resistance. Further, Al reacts with SiC to form an alloy and unevenly erodes SiC. As a result, the surface of the electrode becomes rough, and wiring to the outside is difficult. In order to form a low-resistance ohmic electrode on a p-type SiC semiconductor, a method of subjecting a semiconductor region directly below an electrode to a heavy doping treatment to reduce the thickness of a Schottky barrier is effective as in the case of other p-type wide-band-gap semiconductors. However, in the DA method according to the related art, since an interface reaction between a semiconductor substrate and a deposited film is utilized, the semiconductor region directly below the electrode subjected to the heavy doping treatment is consumed in the interface reaction.
As described hitherto, according to the related art, an electrode demonstrating low smoothness or low ohmic properties is obtained. Although the smoothness is slightly improved by lowering the temperature of the thermal treatment, the interface reaction does not progress, and only an electrode demonstrating low ohmic properties and high contact resistance is obtained. Thus, the inventors applied for a patent on an ohmic electrode having an ohmic electrode layer made of Ti3SiC2 and directly laminated on a surface of a p-type SiC semiconductor and a method of forming the ohmic electrode, in the form of Japanese Patent Application No. 2009-020850.
As a result of further studies, the inventors found out that although an ohmic electrode demonstrating good ohmic properties is obtained according to the invention described in the aforementioned patent application, the composition ratio needs to be strictly controlled. In addition that the inventors found out that there is a crystal grain boundary in the electrode layer because the electrode layer has a polycrystalline structure, and the electrode may be corroded due to penetration of chemicals into the crystal grain boundary if the surface of the electrode is exposed in a step following formation of the electrode.